Wireless communication systems are widely deployed to provide various types of communication; for instance, voice and/or data may be provided via such wireless communication systems. A typical wireless data system, or network, provides multiple users access to one or more shared resources. A system may use a variety of multiple access techniques such as Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), Code Division Multiplexing (CDM), and others.
Examples of wireless systems that enable various types of communication include Wireless Local Area Networks (WLANs) such as WLANs that comply with one or more of the IEEE 802.11 standards (e.g., 802.11(a), (b), or (g)). Additionally, IEEE 802.11(e) has been introduced to improve some of the shortcomings of previous 802.11 standards. For example, 802.11(e) may provide Quality of Service (QoS) improvements.
The IEEE 802.11n standard for wireless communications, expected to be finalized in mid-2007, incorporates multiple-input multiple-output (MIMO) multiplexing into the orthogonal frequency-division multiplexing (OFDM) technology adopted by previous versions of the 802.11 standard. MIMO systems have the advantage of considerably enhanced throughput and/or increased reliability compared to non-multiplexed systems.
Rather than sending a single serialized data stream from a single transmitting antenna to a single receiving antenna, a MIMO system divides the data stream into multiple unique streams which are modulated and transmitted in parallel at the same time in the same frequency channel, each stream transmitted by its own spatially separated antenna chain. At the receiving end, one or more MIMO receiver antenna chains receives a linear combination of the multiple transmitted data streams, determined by the multiple paths that can be taken by each separate transmission. The data streams are then separated for processing, as described in more detail below.
In general, a MIMO system employs multiple transmit antennas and multiple receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS eigenmodes corresponding to independent virtual channels, where NS≦min{NT, NR}.
In a wireless communication system, data to be transmitted is first modulated onto a radio frequency (RF) carrier signal to generate an RF modulated signal that is more suitable for transmission over a wireless channel. For a MIMO system, up to NT RF modulated signals may be generated and transmitted simultaneously from the NT transmit antennas. The transmitted RF modulated signals may reach the NR receive antennas via a number of propagation paths in the wireless channel. The relationship of the received signals to the transmitted signals may be described as follows:SR=HST+n  Eq. (1)where SR is a complex vector of NR components corresponding to the signals received at each of the NR receive antennas; ST is a complex vector of NT components corresponding to the signals transmitted at each of the NT transmit antennas; H is a NR×NT matrix whose components represent the complex coefficients that describe the amplitude of the signal from each transmitting antenna received at each receiving antenna; and n is a vector representing the noise received at each receiving antenna.
The characteristics of the propagation paths typically vary over time due to a number of factors such as, for example, fading, multipath, and external interference. Consequently, the transmitted RF modulated signals may experience different channel conditions (e.g., different fading and multipath effects) and may be associated with different complex gains and signal-to-noise ratios (SNRs). In equation (1), these characteristics are encoded in matrix H.
In many wireless communication systems, one or more reference signals, known as pilot tones, are transmitted by the transmitter to assist the receiver in performing a number of functions. The receiver may use the pilot tones for estimating channel response, as well as for other functions including timing and frequency acquisition, data demodulation, and others. In general, one or more pilot tones are transmitted with parameters that are known to the receiver. By comparing the amplitude and phase of the received pilot tone to the known transmission parameters of the pilot tone, the receiving processor can compute channel parameters, allowing it to compensate for noise and errors in the transmitted data stream. Use of pilot tones is discussed further in U.S. Pat. No. 6,928,062, titled “Uplink pilot and signaling transmission in wireless communication systems,” the contents of which are incorporated herein by reference.